1. Field of the Invention
This invention relates to projection lens systems for use in projection televisions and, in particular, to improved projection lens systems having both a wide field of view, e.g., a semi-field of view on the order of about 40.degree., and a low f-number, e.g. , a f-number on the order of about 1.0 or less.
2. Description of the Prior Art
Projection lens systems for CRT projection televisions have undergone continuing development during the past fifteen years or so. As a result, many of today's CRT projection televisions are equipped with fast lens systems having f-numbers of f/l and smaller and provide semi-fields of view of 25.degree. and wider.
As is well-known, color images are obtained for television systems of this type by combining images from three color CRTs, i.e., a red CRT, a green CRT, and a blue CRT. Since the emission spectra of the CRTs are relatively narrow, for consumer applications, lens systems uncorrected for color can often be used.
When the field of view does not exceed a half-angle of about 25.degree., the lens system may consist of just three components and still provide a sufficiently high level of image quality. A typical configuration has a weak aspherical element on the image side of the lens system, followed by a strong positive power element, followed in turn by a strong negative element in close proximity to the CRT. See, for example, Betensky, U.S. Pat. Nos. 4,300,817, 4,348,081, and 4,526,442.
For this configuration, the aspherical first element provides most of the correction of spherical aberration and other aperture dependent aberrations, the positive element in combination with the relative position of the aperture stop of the lens system allows for the astigmatism to be corrected, and the negative element close to CRT provides correction for the field curvature of the lens.
When the focal length of the lens system is shortened to make the projection television more compact, the field coverage of the lens system must be increased. When the half-angle of view is increased to about 28.degree., a three element form generally cannot provide an appropriately high level of optical performance. To address this problem, a fourth element (corrector element) has been added between the strong positive and the strong negative elements of the three component configuration. See Betensky, U.S. Pat. No. 4,697,892, and Moskovich, U.S. Pat. Nos. 4,682,862, 4,755,028, and 4,776,681. This additional element usually does not have much optical power; however, it must have an aspherical surface to correct for aperture dependent off-axis aberrations like sagittal oblique spherical and coma. Four element configurations have been used effectively up to half-angles of 33.degree. to 35.degree.. Five element configurations are also known in the art. See the Moskovich '862 and '681 patents, supra.
One of the most widely used CRT projection television configurations is the rear projection TV. In this configuration, the CRT image is projected onto the back side of a diffuse screen which is viewed from its front side by the TV audience. To make the image bright and evenly illuminated, most projection screens have employed fresnel lenses and lenticular lenslet arrays. As a result, these screens have typically had a numerical aperture of about 0.57, corresponding to a maximum half field of view of the lens system of about 35.degree.. Improvements in screen manufacturing technology have allowed the maximum numerical aperture to be increased to about 0.65 corresponding to a maximum half field of view of the lens system of about 40.degree.. This increased field of view should allow for significant reduction in the size of projection TV sets. However, it has been found that the four and five element projection lens system configurations described above do not in general provide sufficiently high image quality when used at semi-field angles of 40.degree..